In certain video disc systems video information is recorded by means of geometric variations in the bottom of a smooth spiral groove on the surface of a disc record. The disc record surface includes a coating of conductive material which is preferably covered with a thin deposit of dielectric material. A signal pickup, supported at one end of a pickup arm, engages the spiral groove and includes a conductive electrode which establishes a capacitance with the conductive coating and the dielectric deposit of the disc record. When the disc record is rotated, the pickup/record capacitance varies in response to the geometric variations in the bottom of the spiral groove. The capacitance variations are converted to electrical signal variations by suitable signal processing circuitry coupled to the pickup electrode. The output signal of the signal processing circuitry may be coupled to a conventional television receiver for reproduction. The other end of the pickup arm is releasably secured to a support member of a supporting structure of the playback system. A system of the aforementioned type is described in detail in U.S. Pat. No. 3,842,194, issued to J. K. Clemens on Oct. 15, 1974, and entitled " Information RECORDS AND RECORDING/PLAYBACK SYSTEMS THEREFOR".
Video disc systems of the aforementioned type generally utilize disc records having groove densities in the order of four to eight thousand groove convolutions per inch. A typical video disc record of this type may have a groove convolution spacing in the order of 3.5 microns. The fragile walls of relatively narrow grooves of the disc record cannot be dependably relied upon to pull the weight of the pickup arm assembly, around the pickup arm pivot support, across the entire recorded surface of the disc record. Also in video disc systems utilizing variable capacitor concept, it is desirable for accurate reproduction of the prerecorded signals that the signal pickup electrode maintain a substantially constant attitude in the spiral groove. Therefore, the supporting structure includes a radial feed drive mechanism for traversing the supported end of the pickup arm in proper time relationship with the radial motion of the signal pickup tip engaged in the spiral groove so as to continuously maintain the longitudinal axis of the pickup arm substantially tangential to the spiral groove at the point of engagement. Reference may be made to the U.S. Pat. No. 3,870,835, issued to F. R. Stave, and entitled "VIDEO DISC PLAYBACK APPARATUS", for an illustration of a suitable radial feed drive mechanism for providing the indicated radial motion.
Further, in the above-mentioned type video disc systems it has been recognized that the relative motion between the disc record and the signal pickup should be maintained at a predetermined speed, and within specified tolerance limits (e.g., 450 rpm, .+-.0.01 percent), in order to obtain high fidelity of reproduction of the prerecorded signals. The predetermined speed and the specified tolerance limits are also necessary to assure that the horizontal and vertical synchronizing information is stable and within the lockup range of the deflection circuits of the television receiver. Moreover, when the prerecorded information is a color television signal with chrominance information recorded as a modulated carrier signal, the recovered signal must be stable and within the lockup range of the color processing circuits of the playback system in order to reduce color phase distortion.
The disc record/signal pickup relative speed may be maintained at the predetermined speed, and within the specified tolerance limits by rendering the pickup arm support member subject to cyclical, translatory motion along the longitudinal axis of the pickup arm in a manner that opposes deviations of the instantaneous relative speed from the predetermined speed. Illustratively, the means for imparting translatory motion to the support member may be of the type disclosed in the U.S. Pat. No. 3,711,641, issued to R. C. Palmer on Jan. 16, 1973, and entitled "VELOCITY ADJUSTING SYSTEM".
It may be noted that the disc records having high groove densities (e.g., 4,000 to 8,000 grooves per inch) are subject to occasional flows causing premature termination of the spiral groove. Such premature termination (commonly referred to as a "locked groove") may result in an undesired repetition of a particular groove convolution during disc record playback. It is therefore desirable to incorporate in a video disc player an apparatus suitable for relieving a signal pickup from a locked groove condition, when encountered.
In copending applications of (1) M. A. Leedom et al., Ser. No. 499,557, filed Aug. 22, 1974, entitled "DISC RECORD GROOVE SKIPPER", and (2) H. N. Crooks, Ser. No. 522,818 now U.S. Pat. No. 3,963,861, filed Nov. 12, 1974, entitled "DISC RECORD GROOVE SKIPPER APPARATUS", both assigned to the present assignee, arrangements for repositioning a signal pickup from one convolution to another of a spirally grooved disc record are described. The above-said arrangements employ a bimorph type element for producing special effects (e.g., stop action, fast forward, slow motion, etc.) and for relieving the signal pickup from the locked groove. The present invention is concerned simply with an apparatus for relieving a signal pickup from a locked groove condition, if and when such condition is encountered.